The present invention relates to semiconductor processing techniques, and more particularly to the fabrication of a tiered structure using optical lithography methods and base layer stabilization to prevent interlayer mixing and a method of forming a semiconductor device with the tiered structure.
Gate structures in FET devices are a critical component affecting device performance. Gate structures on many current metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) use metals, such as gold (Au), to achieve the low noise, low resistance performances required. Due to the non-reactive nature of these metals and the difficulty in etching them, additive fabrication techniques, such as evaporative metal deposition and lift-off processing, are typically employed in the manufacturing of these gates.
During operation, the speed of a transistor is inversely related to the length of the gate with smaller gates providing faster switching times. This is because a gate with a small foot or stem (length) offers less gate capacitance. However, one drawback is that such a gate having a conventional rectangular cross sectional area is more resistive as the gate length decreases, because its cross sectional area decreases. Improved gate performance can be attained if a gate is fabricated to have a small length or foot dimension, but a larger top section having a larger cross sectional area. The small foot or stem (gate length) attached to the substrate minimizes gate capacitance, while the larger structure or head on top allows for low gate resistance. This tiered structure now resembles a T and so this type of gate is commonly referred to a T-shaped gate or simply a T-gate. In many instances, this structure is also referred to as a Y-gate or a mushroom gate due to its final shape, which depends upon the method used to deposit the metal. Deposition methods such as evaporation or sputtering result in a gate structure which is dimpled at the top and thus take the shape of a Y. Metal deposition which is done from the bottom up such as by plating, results in a gate structure which is more rounded in profile, thus resembling a mushroom. In either case, the cross-sectional dimension is not uniform and thus is deemed a tiered structure. In yet another instance, a tiered structure known as a gamma-gate can be produced. A gamma-gate is fabricated to include a top tier which is not centrally aligned with the bottom tier, or stem portion. This can be easily done by purposeful and controlled misalignment during lithography of the top tier with respect to the bottom tier. Accordingly, use of the terms T-gate, Y-gate and gamma-gate are all synonymous terms, since all apply to tiered structures having slightly different cross-sectional shapes. In this disclosure the term T-gate, the most common term in the art, is intended to encompass all of these structural variations.
Typically, T-gate structures are used in high performance devices, as they give lower noise performance, higher gain, and higher cutoff frequencies as compared to simple rectangular gates. In a metal T-gate the upper, wider part of the gate increases the cross-section of the gate, which reduces the gate resistance. So where a small gate length reduces noise, a T-gate with a small gate length reduces noise even more.
Fabrication of a metal structure such as a gate using an additive metallization process, requires that a liftoff resist process be used. In a liftoff resist process, special techniques and resist materials are used to form a retrograde or re-entrant resist profile. Here the top of the resist is opened more narrowly than the bottom of the resist which is processed to open more widely, forming the desired profile. This is in contrast to conventional resist processing such as when resist is used as an etch mask. Here, it is advantageous that the resist profile be either vertical or have a slightly positive slope. There are many known resist processes used for liftoff fabrication, but all have in common this retrograde profile. The most commonly used processes employ two different resist materials which are layered one on the other to form a bilayer stack. The two materials have different exposure and dissolution characteristics which cause each to have a different development rate. The bottom layer is generally a somewhat isotropically developing, low contrast resist and is made to develop laterally more than the top layer. The top layer is a more anisotropically developing, high contrast layer used to define the opening for metal deposition and ultimately define the metal structure""s dimension (length). Other processes have made clever use of resist chemistry and have achieved a similar retrograde profile in only a single layer. This is done by controlled diffusion of a neutralizing agent into the top surface of the resist to a limited depth. This neutralizing agent (typically a base) slows the dissolution rate of the resist near the top of the resist layer where the concentration of the agent is greatest. Dissolution rate is unaffected below a certain depth into the resist, where development occurs at a normal rate. The slowed rate for the top compared to the bottom portions of the resist layer combine to create a retrograde profile.
A T-gate is a special type of structure, and like conventional rectangular gates, can be fabricated using additive metallization and liftoff processing. However, since a T-gate is an example of a tiered structure, the resist process is more complicated since an additional tier of resist is needed to form the stem of the T shape. As with conventional liftoff processing, many types of T-gate processes have been proposed. All have in common a high resolution opening in the base resist layer used to form the narrow part, the stem. A conventional bilayer or single layer liftoff resist process is built on top of the base layer which forms the wider, current flowing, top portion of the gate. Once the complete stack is built, metal is deposited into the opening and fills the structure starting from the bottom (base or stem section) and proceeds through the top section, stopping when the desired thicknesses are achieved.
Electron-beam lithography has been often used for the fabrication of T-gate resist profiles. E-beam offers the highest resolution possible, a factor which is necessary for creation of small gate lengths which play a key role in gate performance. However, many other factors are beneficial and naturally inherent to e-beam lithography, and are in contrast, absent from optical lithography. Such factors add additional favor to e-beam lithography as a means of fabricating T-gate resist profiles. The two most important of these are, the transparency of e-beam resists to high energy electrons, and the insolubility of e-beam resists which allow for multi-level stacking.
Polymeric materials used as e-beam resists are largely transparent to high energy electrons used for exposure. For this reason, it is well known in the art that a single e-beam exposure, coupled with the correct tri-layer resist stack and selective developers is adequate to expose a completed T-gate resist profile. This is not true of other types of lithography such as optical lithography where resist materials are largely absorbing making it impossible to pattern, with the highest resolution, the base layer through the adjacent overlayers. In addition, many commonly used e-beam resists such as PMMA (poly methylmethacrylate) have a very high molecular weight (MW) (often greater than 1,000,000) and achieve development contrast through e-beam induced chain scissions which dramatically reduce MW in exposed areas. Rapid development of exposed areas occurs since lower molecular weight polymers dissolve much more rapidly than high molecular weight polymers. The relative insolubility of unexposed high MW e-beam resists also allows them to be coated in adjacent laminar layers without interlayer mixing or solvent attack from the next layer to be coated. This low solubility is a key enabling factor which allows a complicated T-gate liftoff resist stack to be built. Optical resists are not chain scissioned upon exposure, but chemically react to form more soluble products. However, even unexposed, they are highly soluble compared to conventional e-beam resists due in part to their relatively low MW ( less than 100,000 typically), and thus are subject to intermixing and solvent attack from subsequent layers. It should be pointed out that while the molecular weight of a polymer affects its solubility, structural features which determine the polarity of the polymer also play a role.
For these reasons conventional novolak/DNQ (diazonapthoquinone) or chemically amplified optical resists are not used for T-gate fabrication. The one exception to this is the case where a negative acting resist is patterned as the base layer using optical methods, and the cap portion is created by an alignment and exposure of a second mask. This works since this type of negative resist, is by its nature crosslinked. Its crosslinked structure gives it a very high MW and low solubility. Such a layer used as the base of a T-gate is unaffected by subsequent resist layer applications. Other methods of fabricating T-gates which do not use resist materials as base layers have been proposed. Using these methods, T-gates are created by first forming the stem by etching into a film, such as oxide or nitride. This is followed by a bi-layer lift-off process to create the cap section.
However, all of the above methodologies are inadequate when proposed for use in a manufacturing environment where simplicity and reliability are required. E-beam tools, while offering high resolution, are very slow and have extremely poor throughput. Using an oxide or nitride process to form the stem adds many steps to the fabrication process, and is inherently inefficient. These additional materials also create additional undesirable device parasitics such as increased gate capacitance which detract from device performance. In addition, negative resists are not widely accepted by industry and thus are not used in modern fab environments; only positive resists are used. Therefore, the most valuable process for forming T-gates is one which would use only modern high throughput optical lithographic methods, tooling, and materials to form the entire required resist structure. This can only be done using modern positive i-line and chemically amplified deep ultra violet (DUV) resists, with a technique that is also extendable to next generation lithography tooling and resist materials such as 193 nm, 157 nm, 126 nm and Extreme Ultra Violet Lithography (EUVL).
Accordingly, it is an object of the present invention to provide for a method of fabricating a tiered structure, such as a T-gate structure, utilizing optical lithography methods.
It is another object of the present invention to provide for a method of fabricating a tiered structure in which a stabilized resist base layer is fabricated to prevent interlayer mixing during subsequent layer fabrication.
It is yet another object of the present invention to provide for a method of fabricating a tiered structure in which standard and conventional resist materials are utilized without the sacrifice of any resolution needed to form the structure.
It is still a further object of the present invention to provide for a method of fabricating a tiered structure utilizing positive resists composed of moderate molecular weight, linear polymer chains.
These needs and others are substantially met through provision of a method for fabricating a tiered structure, such as a T-gate structure, including the steps of providing a substrate and depositing on the uppermost portion of the substrate, a plurality of organic resist layers. A first resist layer is deposited on the substrate and a trench for a gate stem is formed in the layer, by imaging with a radiation source. The resist layer undergoes a stabilization process in which radiation and/or heat are applied to the layer, thereby causing the layer to react and cross-link without destroying or deforming the trench formed for the gate stem. Next, a resist stack is deposited on the stabilized layer. Due to the stabilizing of the first resist layer, there is no deforming effect on this layer upon the introduction of the resist stack. The resist stack is next aligned, exposed and developed thereby providing for the opening of a large trench over the trench formed for the gate stem. A re-entrant profile is formed in a portion of the resist stack adjacent the stabilized resist layer. A metal is deposited in the formed trench opening, and the organic layers are subsequently removed to expose the tiered structure.